This invention is related to wireless networks. One aspect is a method for a wireless station of a wireless network to select an access point according to a received signal quality measure. One version uses a measure of the error vector magnitude (EVM).
Wireless networks such as wireless local area networks (WLANs) have recently become popular. A WLAN may be an ad-hoc network in which any wireless station (STA) may communicate directly with any other STA, or an infrastructure network in which one STA acts as an access point (AP). All other STAs of the network associate with the AP, and communicate only via the AP. The AP may be connected to other networks by a wired or wireless connection.
The description herein will assume a wireless network that conforms to the IEEE 802.11 standard, and will use the terminology of the IEEE 802.11 standard. The invention, however, is not restricted to such a network.
A station of a wireless network, i.e., a STA, includes a physical layer processor (PHY) and a MAC processor. An AP is a STA that transmits messages (beacons or probe responses) that provide information (PHY and MAC information) for other stations that enable such other stations to associate with the AP. Beacons or probe responses are similar except that a beacon is broadcast not necessarily in response to any external event, and a probe response is transmitted in response to the AP receiving a probe request message. For any particular STA that desires to associate with an AP, there may be several APs with which to associate. STAs often scan for APs with which they can associate and desire to associate with the “best” AP.
A STA that wants to operate as a client station in an infrastructure network, also called a basic service set (BSS), will usually attempt to identify all the APs with which it can associate by scanning one or more channels, e.g., for beacons and probe responses. The STA will so scan at start-up and periodically thereafter.
An AP that acts as a repeater AP will also scan one or more channels to identify potential parent APs, although usually only on start-up or if its parent AP is no longer available.
The scanning may be active or passive. In a “passive scan”, a STA listens for beacons from APs on one or more channels. In an “active scan”, a STA sends a probe request message and listens for probe responses in response to the probe request, on one or more channels.
A STA may also record other packets from an AP in addition to beacons and probe responses.
Scans provide information at the MAC level such as the data rates supported by the AP, the identifier of service set (the SSID), security parameters for communicating with the AP, the load at the AP, and so forth. Scans also provide PHY layer information. In particular, when a STA receives the beacon or probe response, the STA records the received signal strength indication (RSSI) at the PHY of the receiving STA.
The information provided by scanning, after appropriate weighting, is often used to determine the “best” AP with which an association should be attempted.
The IEEE 802.11a PHY standard defines RSSI as a measure by the PHY sublayer of the energy observed at the antenna used to receive the packet. RSSI is measured by the PHY during packet reception and is passed up with the packet. The RSSI is often used to differentiate the signal strength from candidate APs and to determine the “best” AP according to the received signal strength when all other measures, such as loading, etc., are equal.
Those skilled in the art will recognize that the RSSI is a measure of signal strength but not signal quality. It has been found that the RSSI is not a good indicator of the signal quality or a good measure for “best” AP selection. This may be for a variety of reasons. For example, when scanning, often the only packets available for a STA to measure are sent at a relatively low rate. This is particularly true if RSSI values for AP selection are only available from a beacon or a probe response, which are usually sent at a low rate. The RSSI value from a low rate packet only provides a very coarse indication of how well the link will support high rate packets. Such an RSSI value, for example, does not account for factors that significantly reduce signal quality such as multipath or the presence of strong interferers.
Using the RSSI to select the “best” AP can result in a lower throughput and latency than would occur if a measure of signal quality rather than signal strength was used for the selection. Selecting the AP using the RSSI can also lead to frequent roaming, and hence instability. These are particular issues for voice and other applications that require high throughput or low latency.
Thus there is a need in the art for a method of selecting an AP for association based on a measure indicative of the received signal quality and of the quality of communication achievable on the link between the AP and a client station.